This invention relates to polyolefin compositions having a stability against degradation caused by exposure to heat and oxygen. Specifically it relates to a synergistic combination of stabilizers which make the compositions ideally suited as insulation for underground electrical cables.
It is well known in the art to stabilize polymers against degradation due to heat and oxidation by incorporating into the polymers a stabilizing amount of hindered phenolic antioxidants such as those disclosed in U.S. Pat. No. 3,285,855. However a serious disadvantage to the use of plastic materials such as polyolefin resins, as insulation for wire and cables is the fact that the degradation of the polymer is accelerated by the presence of copper and alloys of copper. Prior art methods to solve this problem have resulted in a combination of the polyolefin resins with the primary hindered phenolic antioxidant and a copper deactivator or chelating agent such as certain organic hydrazide or hydrazine compounds, e.g., those disclosed in U.S. Pat. Nos. 3,438,935, 3,484,285 and 3,772,245.
It is also well known in the prior art that the stabilizing efficiency is improved by the addition of synergists such as dilauryl thiodipropionate (DLTDP) or distearylthiodipropionate (DSTDP). In summary, in order to produce a polyolefin based resin composition which is acceptable for use in coating of cables, wires and other electrically conducting devices made from copper or copper alloys it is necessary to incorporate in said resin the aforementioned three-component system of antioxidant-chelating agentsynergist. As each of these components are relatively high cost materials and usually added in appreciable quantities, the resultant polyolefin resin compositions have often been deemed not economically feasible for such uses.
It is therefore an object of the present invention to provide a novel stabilized polyolefin resin composition useful as insulation for wire and cables.
It is another object of the present invention to provide a polyolefin resin composition when in direct contact with copper exhibits excellent stability against oxidative and thermal degradation.
It is a further object of the invention to provide a stabilized polyolefin resin composition, wherein one of the components of the stabilizer system functions simultaneously as a synergist and a metal chelating agent.
Other objects of the invention will become apparent from the detailed description and appended claims.